1. Field of the Invention
Apparatus and methods according to the present invention relate to methods for compensating a sampling frequency offset and an Orthogonal Frequency Division Multiplexing (OFDM) signal receiving apparatus thereof. More particularly, they relate to methods for compensating a sampling frequency offset for preventing Inter Symbol Interference (ISI) from occurring in a receiving apparatus which is used in OFDM communicating systems such as a terrestrial Digital Multimedia Broadcasting (DMB) systems, and a OFDM signal receiving apparatus thereof.
2. Description of the Related Art
FIG. 1 is a block diagram of a general OFDM receiving apparatus. With reference to FIG. 1, the OFDM receiving apparatus comprises an antenna 10, a tuner 20, an analog-to-digital computer (ADC) 30, a demodulating unit 40, a decoder 50, a reproducing unit 60, and a synchronizing unit 70.
An analog OFDM broadcasting signal in a specific frequency band that is tuned by the tuner 20, among OFDM broadcasting signals fed through an antenna 10, is converted into a digital OFDM broadcasting signal by the Analog to Digital Converter (ADC) 30. Then, the demodulating unit 40 demodulates the digital OFDM broadcasting signal by implementing a Fast Fourier Transform (FFT), sub-carrier demodulation, deinterleaving, channel decoding, etc. on the digital OFDM broadcasting signal that was converted by the ADC 30. At this time, the synchronizing unit 70 may implement a function for compensating for a sampling frequency offset that is used by the demodulating unit 40 to implement the FFT. The OFDM broadcasting signal demodulated by the demodulating unit 40 is decoded into an original image signal by means of MPEG decoding by the decoder 50, and then reproduced by means of the reproducing unit 60.
FIG. 2 is a drawing that illustrates an influence on a signal due to the sampling frequency offset applied to the FFT. In FIG. 2, “(a) no offset” illustrates a case where a sampling frequency offset does not occur, “(b) positive offset” illustrates a case where a positive sampling frequency offset occurs, and “(c) negative offset” illustrates a case where a negative sampling frequency offset occurs.
As can be known in FIGS. 2 (b) and (c), the occurrence of the sampling frequency offset results in an FFT interval transition. If the sampling frequency offset is small so that the FFT interval transition is relatively small, the offset does not become a problem. However, if the sampling frequency offset is large so that the FFT interval transition is relatively large, ISI may take place.
Meanwhile, if the duration of a channel delay profile is so long that it nearly comes close to the duration of a guard interval included in OFDM symbols, it is more likely that ISI occurs. FIG. 3 exemplifies a channel delay profile having a duration of 200 μs. If it is assumed that FIG. 3 shows a profile for a local network, it is obvious that the duration of the channel delay profile for a single frequency network becomes longer than that of FIG. 3.
In view of such facts, given that the guard interval has a duration of ‘246 μs’, the duration of the channel delay profile may nearly come close to that of the guard interval. Therefore, there is an immediate need to compensate for the sampling frequency offset.
Conventionally, there was universally used a method for compensating for a sampling frequency offset by means of sampling frequency stabilization using a Voltage Controlled Crystal Oscillator (VCXO).
However, since a VCXO is expensive and has a high power consumption, it implies the problem of the OFDM receiving apparatus being expensive and having a high power consumption. Accordingly, if an inexpensive general oscillator is used instead of the VCXO, a method for compensating for the sampling frequency offset is needed.